Thanks for this!
Gallery Forum attachments and Regular Forum post attachments store their files in different directories. They do this because gallery attachments are stored in three different sizes, where as Regular Forum attachments are stored uncompressed and on touched from what the original uploader had intended for them to be.
I see the bug that you're pointing out.
"Gallery Post attachments" vs "Regular Post attachments." Each post attachment type is determined by the forum type which it resides in. Making a post global, ignores what type of forum the original post was created in, and tries to display that post's attachments as whatever type of forum the user is current viewing the global announcement within.
A) The /simple/ solution to this would be to make your Global Announcement within a regular post forum, and include a link to your gallery from there. Basically not crossing over the two different types.
B) From a programming standpoint (possible solution for a future release), a "click" on a global announcement should take a user in to the correct forum where the post was created within, and display correctly using those perimeters.
Thanks for this good report - I've bookmarked it and will look in to resolving it for a future release.